The hydrogen peroxide oxidation of dihydroquinacridones, as described in U.S. Pat. No. 5,840,901, is a new environmentally friendly method for preparing quinacridone pigments. This process, like other oxidation methods, has one drawback in that it produces mostly large particle size crude products. Any improvement of this method to produce small particle size pigments would be highly desirable as this would eliminate the need for costly and undesirable finishing processes such as dry milling (U.S. Pat. Nos. 2,402,167 and 3,030,370) and acid drowning (U.S. Pat. Nos. 3,326,918 and 3,607,336).
It is well known that the unsubstituted quinacridone, depending on the experimental conditions, forms different crystal phases such as alpha, beta and gamma including the gamma-I, gamma-II and gamma-III polytypes. The currently commercial and important quinacridones are beta, gamma-I and gamma-II crystal forms. The gamma-I and the gamma-III crystal phase quinacridone can be produced by either recrystallizing in a particular solvent or by a dry milling process.
The use of derivatized pigments as additives in pigmentary quinacridone preparation is well known. Some of the already known quinacridone derivatives are e.g. pyrazolylmethyl quinacridone; phthalimidomethyl quinacridone; quinacridone sulfonic acids; various salts of the quinacridone sulfonic acids and many others. Some of these quinacridone derivatives have been used as pigment particle growth modifiers and some others are used for rheology improvement. The known methods for the preparation of pigmentary quinacridones are for instance the milling of large particle size crude quinacridones in the presence of large amounts of an inorganic salt which must be removed afterwards as described in U.S. Pat. No. 5,084,100. Pigmentary quinacridones can be obtained by a tandem premilling-solvent ripening method as described in U.S. Pat. Nos. 5,281,269 and 4,541,872.
The above mentioned additives for instance have been used during the pigment preparation by the milling, solvent treatment, or post-solvent treatment steps to further improve pigment properties. For example, U.S. Pat. No. 5,084,100 discloses a method in which crude quinacridone is ball milled in the presence of aluminum sulfate and the dibasic esters of various carboxylic acids. Other ways of making pigmentary quinacridones are described in U.S. Pat. No. 4,455,173 wherein the crude quinacridone pigments are acid pasted or ball milled and then milled in an organic liquid, preferably in the presence of a 2-phthalimidomethylquinacridone particle-size growth inhibitor. Various pigment derivatives for treating pigments, including quinacridone pigments, have also been described in U.S. Pat. Nos. 3,386,843; 4,310,359, and 5,472,494.
The addition of certain quinacridone derivatives in the preparation of quinacridones by the polyphosphoric acid ring-closure route has been reported in U.S. Pat. Nos. 5,368,641 and 5,755,873 which disclose the preparation of pigmentary quinacridones, particularly the pigmentary 2,9-dimethylquinacridone. The use of such pigment derivatives in the preparation of pigmentary quinacridones by the oxidation of dihydro quinacridones has been reported in U.S. Pat. Nos. 5,424,429, 5,457,203 and 5,840,901.
The preparation and uses of the triazine UV absorbers have been described in patent literature. These additives are used for automotive coatings, photographic application, polymeric film coatings and ink jet printing. Automotive coatings are described in British 2,317,174A and 2,317,893A and in U.S. Pat. Nos. 5,354,794; 5,556,973; 5,681,955; 5,726,309 and 5,106,891. Photographic applications are disclosed in U.S. Pat. No. 3,843,371. Polymeric film coatings are described in U.S. Pat. Nos. 4,619,956 and 4,740,542. Ink jet printing is disclosed in U.S. Pat. No. 5,096,489. Tris-aryl-s-triazines are commonly understood to refer to tri-aryl compounds in which at least one of the aryl groups has a hydroxy group ortho to the point of attachment to a triazine ring. Resorcinol derivatives, for purposes of this application, are compounds substituted in the 2,4-positions by hydroxyl groups on at least one phenyl ring of a tris-aryl-s-triazine compound. U.S. Pat. No. 5,726,309 describes resorcinol numbering and is incorporated herein in its entirety by reference.
Accordingly, it was the object of the present invention to provide a process for producing alpha, gamma-I, gamma-II and gamma-III crystal modifications of quinacridone and transparent quinacridone pigments including the solid solutions of various quinacridones.
Said object has surprisingly been solved by a process for producing a quinacridone pigment according to formula B 
or a solid solution thereof,
comprising oxidizing a salt of a corresponding 6,13-dihydroquinacridone of formula A or a mixture of two or more 6,13-dihydroquinacridones of formula A 
in which X and Y are independently 1 or 2 substituents selected from the group consisting of H, F, Cl, C1-C4 alkyl and C1-C4 alkoxy, in the presence of at least one (hetero)aromatic hydroxyl or keto group containing compound that does not contain a pigment moiety and an oxidizing agent.
That is, this invention relates to a process for obtaining alpha, gamma-I, gamma-II and gamma-III quinacridones and other transparent quinacridones and their solid solutions involving various compositions of structure B, using the hydroxyl or keto group containing aromatic and heteroaromatic compounds, for instance, resorcinol triazine compounds, as additives in the oxidation, for example the hydrogen peroxide oxidation of dihydroquinacridones. The additives are used at 1-25% by weight relative to the weight of dihydroquinacridones, preferably at 1-10%.